Monday, May 30, 2011

A bumper 'crop'

We returned to Australia on 31 December and I harvested the wheat a week later. It came in at 4.95 tonnes per hectare – a very respectable yield! All I did was sow, weed once and then harvest six months later.

Of course it was an extraordinarily favourable season, although I was surprised to get such a high yield with no added fertiliser, especially after very little had been applied to the previous crop. In addition there were 45 days with high potential leaching (see red lines on last post). Unfortunately I have no idea how deep the roots went.

Sunday, May 29, 2011

Water for the wheat crop

During the six month I spent in Africa, I put the whole vegetable garden under wheat and mustard. The bed we are following - where the sweetcorn had been grown - was sown to wheat on 25 April. The graph above gives a snapshot of the soil water regime at a depth of 30 cm from 1 July until harvest date. The topsoil remained wet up to the middle of September, as shown by the blue line (i.e. suction remained under 30 kPa). There were only two times during the season where the topsoil starts to dry – mid October and late November. But in both cases the rain soon returned. In fact it was a very wet season, with the crop receiving over 600 mm (shown on right hand axis).

The horizontal red line shows the period when the wetting front detector at 40 cm depth contained water. In this case the floats were removed from the detector and replaced by a simple electrode which could be logged (A friend downloaded the logger while I was away so I could see when water was collected and later sucked out of the detectors by the drying soil).

The pattern is quite obvious. When there is a lot of rain (the black cumulative rainfall line jumps up), the soil suction falls towards zero (the blue line). During the wettest periods the red line appears, showing the detector contained water. There are seven periods when the detector collected water – some for long periods and some for short. For example the detectors contained water for 109 hours in mid August after receiving 38 mm of rain over 3 days. But in mid October, after 57 mm rain, the detectors only held water for 25 hours. The reason is that the larger plants could suck water out of the soil much faster in the warmer October weather.

Over the six month period the detector at 40 cm depth contained water for a total period of 45 days. These are the periods when water is moving downwards quite quickly, from the topsoil into the heavy clay below. We expect much of the nitrate would be moving down into the subsoil as well.

Friday, May 20, 2011

Nitrate in the fallow

We are going to move fairly quickly through 18 months of soil nitrate monitoring. The green line shows the drop in soil nitrate during the 12 weeks the sweet corn crop was in the ground. We started with 221 kg/ha and this fell to 9 kg/ha at harvest. I hope most of this nitrate was picked up by the crop, but no doubt some escaped below the root zone.

Lesson #1 is don’t necessary believe the recommendations to apply fertiliser at planting. It’s prudent to measure first.

The graph above shows what happened during the 77 days of fallow between the end of the corn crop and the subsequent wheat crop. The soil was still warm during March and April, and there were regular rainfall events. On 23 May the wetting front again reached 40 cm depth and the nitrate had risen from 6 kg/ha to 149 kg/ha. This equates to 143 kg/ha or 13 kg of nitrogen ‘appearing’ in the soil each week. The source of nitrate is the mineralisation of organic matter. In other words soil microbes feeding on the soil organic matter break it back down into plant nutrients. I only measured the nitrate, but the complete suite of plant nutrients would be there too.

Wednesday, May 18, 2011

From drought to flood

This next sequence of blogs is all about nitrogen.

I left Australia to work in Africa in June last year, during the worst drought in living memory. I returned in January 2011 to the worst floods in a quarter of a century.

During the drought I was worried about salt in my soil because I had been using waste water. During the continuous wet weather my attention switched to soil nitrogen. Would it all be leached away?

Before I get into this new story, a quick recap: I started this blog in January 2010 with the following paragraph

“This blog accompanies my book “Out of the Scientist’s Garden”. The book is about how the world uses water in the business of feeding itself. It’s a serious book on a difficult topic, so I wrote in story form, through the lens of my fruit and vegetable garden. I kept the book free of tables and graphs because this type of information is for the specialist, and I was writing the book for everybody interested in water.”

Of course you can’t get away from numbers and graphs if you really want to understand water. Those of you who have followed this blog will see that it is full of such information. I have been running a series of ‘live’ experiments and building up the data sets week by week as the experiments unfold. This makes all the data much easier to follow.

The blog started with a sweetcorn crop grown using water from the washing machine. During the long drought and accompanying water restrictions, many people gave up on gardening all together. We maintained a fully irrigated and productive food garden whilst using less water than the average per capita consumption for our city. To keep this up we had to exploit every opportunity, and the challenge of waste water was one of those opportunities.

Water from the washing machine contains salt and plants don’t like salt. The first of the ‘live’ experiments was about monitoring how the salt was building up in the soil. One of the surprises from this experiments (and all good experiments should have surprises) was how much nitrate was in my soil, even before any compost, manure or fertiliser was added.

Plants need nitrogen as it is the essential component of enzymes, particularly the photosynthetic enzyme packed into the leaves. Most of the nitrogen the corn plant uses comes in the form of nitrate dissolved in the soil water. You will recall that wetting front detectors were placed at 20 and 40 cm depth in the soil. With a few assumptions I’m going to chart the level nitrate in the soil from the very start (Dec 09 to now May 11).

The assumptions are
1) I take the water sample from the detector at 40 cm depth to be the average value over the top 50 cm of soil
2) I assume the soil is 35% volumetric water content when the soil water sample is captured
3) I use the ‘RQEasy’ meter to read the nitrate level off a colour test strip
4) The value of nitrate in mg/L is converted into kg/ha.

Here is the corn crop that would have taken up approximately 200 kg of nitrogen on a per hectare basis. We only added the equivalent of 20 kg. The rest came from the soil. You can see all the details by scrolling back to the very beginning. In following blogs we will look what happens to the soil supply of nitrate over the next 3 crops.